In sport fishing, one of the challenges is to land a fish using a line of relatively light weight with respect to the weight of the fish. The term "weight" of the line is a term specifying its strength, usually expressed as "n" pounds test line, where "n" is the number of pounds of tensile strength of the line. Typically, fishing line can have a tensile strength in the range of approximately 2-50 pounds.
One way to protect the line against breakage is to utilize a fishing reel incorporating a drag assembly. In some such reels, the drag assembly provides an interface between a spool carrying the fishing line and a hand operated shaft and crank for turning the spool. In fly fishing reels, the drag assembly acts between a frame and a spool, with the handle for turning the spool being a part of the spool. The object is to adjust the drag so that the spool will slip relative to the shaft and crank or frame and thereby protect the line from breaking by releasing the line from the spool.
It will further be appreciated that in crank type reels, the drag system provides for the unwinding and release of line without having to rotate the shaft and crank assembly. If a large fish, full of fight, is hooked, the fish may dive or run and thereby unwind line rapidly from the spool. Typically, such unwinding is too rapid for the fisherman to keep up through operation of the crank or handle. Releasing the crank or handle in such a situation is a poor option, as the reel then provides no resistance to unwinding, when such resistance is necessary for fighting and tiring the fish. Also, if the crank were released to permit the line to unwind, it would be difficult to regain control of the crank until such time as the fish slowed or stopped.
It will also be appreciated that the drag assembly limits the force that can be applied to the fish through the line as the fisherman reels in the fish or flexes the fishing rod. The drag assembly provides for slippage of the spool relative to the shaft and crank or handle, if the fisherman attempts to apply force in excess of the strength of the line or if the fish should suddenly apply such force to the line.
For best fishing results, the fisherman may want to set the drag to utilize the maximum strength of the fishing line without breaking the line. This requires that the drag assembly operate accurately and smoothly. In particular, it is important that the drag assembly operate to provide resistance to unwinding line from the spool, but be unaffected by other aspects of the operation of the reel, such as whether the line is being released from one of the sides or the center of the spool, and, of course, the operation of the drag assembly itself. Most importantly, the drag assembly should be adjustable to cooperate with the strength of the line, but must not inadvertently break the line and lose a large fish.
In fly reels, the spool may be mounted with anti-reverse features, so that it is essential that the drag system be smooth and reliable to avoid line breakage. In fly reels, it is also sometimes desirable to change line, and the ability to quickly change spools would greatly facilitate line changes.